In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imagable elements useful to prepare lithographic printing plates typically comprise one or more imagable layers applied over the hydrophilic surface of a substrate. The imagable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imagable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imagable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital or thermal imaging has become increasingly important in the printing industry because of their stability to ambient light. The imagable elements for the preparation of lithographic printing plates have been designed to be sensitive to heat or infrared radiation and can be exposed using thermal heads of more usually, infrared laser diodes that image in response to signals from a digital copy of the image in a computer a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
Various radiation-sensitive compositions are used in negative-working lithographic printing plate precursors as described for example in U.S. Pat. Nos. 6,309,792 (Hauck et al.), 6,893,797 (Munnelly et al.), 6,727,281 (Tao et al.), 6,899,994 (Huang et al.), and 7,429,445 (Munnelly et al.), U.S. Patent Application Publications 2002/0168494 (Nagata et al.), 2003/0118939 (West et al.), and EP Publications 1,079,276A2 (Lifka et al.) and 1,449,650A2 (Goto et al.).
After imaging, the negative-working lithographic printing plate precursors are developed (processed) to remove the non-imaged (non-exposed) regions of the imagable layer. Often, lithographic printing plate precursors are designed with a water-soluble topcoat or water-soluble oxygen-impermeable barrier layer disposed over the negative-working, photosensitive imagable layer. This topcoat is used to improve high polymerization rate during imaging by assuring higher imagable layer sensitivity.
Such lithographic printing plate precursors are generally transported after manufacture in stacks of dozens or hundreds of individual precursors. Interleaf paper is generally inserted between individual precursors to prevent scratches in the imaging surfaces. However, despite the presence of interleaf papers, the water-soluble topcoat surface can still be scratched during transportation of handling (for example, when removing the interleaf paper in an automated plate loader), leaving to a loss in sensitivity in the scratched areas.
U.S. Patent Application Publication 2010/0151385 (Ray et al.) describes negative-working lithographic printing plate precursors supplied in a stack. Each precursor has a topcoat having a dry coating weight of less than 1 g/m2 so that interleaf paper can be omitted from the stack. Large particles (1-6 μm) are optional relative to the topcoat thickness or dry coating weight (<1 g/m2).
U.S. Patent Application Publication 2007/0231739 (Koizumi) describes negative-working lithographic printing plate precursors in which the protective topcoat contains organic resin fine particles and mica particles in a specified ratio. The organic resin fine particles are preferably in the range of 2-15 μm.
U.S. Patent Application Publication 2007/0231740 (Yanaka et al.) describes negative-working lithographic printing plates in which the topcoat comprises organic filler particles. Similarly, JP 2008-276167 (Yanaka et al.) describes stacks of lithographic printing plate precursors that are separated using interleaf papers. Each precursor has a topcoat comprising silica-coated organic resin particles. Both of these publications describe filler particles at preferably 1-20 μm, more preferably 2-15 μm, and optimally 3-10 μm.
EP 1,865,380 (Endo) describes similar lithographic printing plate precursors having silica-coated polymer particles in the topcoat, which particles have an average diameter of 1-30 μm or even a smaller range of 2-15 μm.
U.S. Pat. No. 5,563,023 (Kangas et al.) describes photoimagable elements having a protective overcoats can contain organic polymeric beads to provide antiblocking properties, which organic polymeric beads have a particle size range of 0.75-75 μm.
The cited publications have a common feature, namely the use of large organic particles relative to the film thickness so that certain space can be maintained when the lithographic printing plate precursors are stacked on top of each other without the insertion of interleaf paper between the precursors. The described large organic particles are ineffective for improving scratch resistance as illustrated in U.S. Patent Application Publication 2007/0231739 (noted above).
There is a need for improved lithographic printing plate precursors having free-radical based negative-working imaging chemistry that exhibit improved outer layer scratch resistance and that can be processed in processors with reduced sludge formation.